1. Field of the Invention
This invention relates to process chambers for chemical vapor deposition and other processing of semiconductor wafers and the like. More particularly, the present invention relates to cold wall process chambers capable of withstanding stresses associated with high temperature, low pressure processes and having improved temperature uniformity and gas flow characteristics.
2. Description of the Related Art
Process chambers for thermally processing semiconductor wafers such as silicon can desirably be made of quartz (vitreous silica) or similar materials which are substantially transparent to radiant energy. Reactors incorporating radiant heat lamps and reaction or process chambers with transparent walls are known in the industry as "cold wall" reactors. Thus, radiant heat lamps may be positioned adjacent the exterior of the chamber and a wafer being processed in the chamber can be heated to elevated temperatures without having the chamber walls heated to the same level. Quartz is also desirable because it can withstand very high temperatures, and because it is inert, i.e., does not react with the various processing gases typically used in semiconductor processing.
Conventional quartz windows used in semiconductor processing chambers generally employ either a flat or outwardly curved configuration. Flat windows are more commonly used when the pressure on the inside of the chamber is substantially the same as the pressure on the outside of the chamber. Flat windows have the advantage of providing a uniform height between the wafer and the inside surface of the window to provide for uniform cross-sections along the flow path of process gases in chemical vapor deposition (CVD), and hence a more laminar flow. Flatwall chambers may also be used when the external pressure outside the chamber differs significantly from the internal pressure within the chamber. However, in such a chamber the windows must be very thick to resist the stresses on the chamber. Thick flatwall chambers unfortunately require additional material and thus add weight to the reactor.
Cold wall chamber designs must also account for thermal effects. In general, the wall temperature during thermal processing should be confined to a very small window. If the temperature gets too high, processing gases can react with one another at the wall (e.g. chemical vapor deposition occurs on the chamber walls). Too low a temperature can cause condensation of constituent gases. In either case, clouding of the walls can cause absorption of radiant heat, leading to cracking and catastrophic failure.
A typical cold wall processing chamber contains a susceptor for supporting the wafer to be processed. This susceptor is often made of a heat absorbing material, which causes the center of the chamber to run extremely hot. When the windows of the chamber are made thick to handle high or low pressure applications, the quartz windows absorb more heat from the inside of the chamber. Additionally, a greater amount of radiant heat is absorbed when passing through thicker transparent walls. Moreover, hotter inner surfaces tend to expand more rapidly than the outer surfaces due to thermal expansion, thereby causing the window to crack.
Forced air cooling is typically applied to the outside of the windows to keep the chamber walls cool during processing. But thick, more massive windows retain more heat, such that forced air cooling is less effective for thick windows. The high temperature at the inner surface of the windows therefore results in chemical deposition on this surface. In addition, it is difficult to direct an appropriate amount of cooling air to a specific location without affecting an adjacent location. Thus, it is difficult to control wall temperature in a desired location to minimize the occurrence of localized depositions.
For applications in which the pressure within a quartz chamber is to be reduced much lower than the surrounding ambient pressure, the strength of the chamber walls becomes important. Dome-shaped chambers have been disclosed, for example, in U.S. Pat. Nos. 5,085,887 and 5,108,792. U.S. Pat. No. 5,085,887 discloses a chamber which includes an upper wall having a convex outer surface and a concave inner surface. A greatly thickened peripheral flange is provided to radially confine the upper wall, causing the wall to bow outward due to thermal expansion, helping to resist the exterior ambient pressure in vacuum applications. The chamber requires a complex mechanism for clamping the thickened exterior flanges of the upper and lower chamber walls.
A lenticular chamber has been described in a pending application entitled PROCESS CHAMBER WITH INNER SUPPORT, Ser. No. 08/637,616, filed Apr. 25, 1996, the disclosure of which is incorporated by reference. This chamber has thin upper and lower curved walls having a convex exterior surface and a concave interior surface in the lateral dimension, with constant longitudinal cross-sections (longitude being defined by the axis internal of gas flow). These walls are joined at their side edges by side rails, thus giving the chamber a generally flattened or ellipsoidal cross-section. The chamber upper and lower walls are generally rectangular in shape, such that a wafer disposed within the chamber is located farther from the upstream and downstream ends than from the lateral side rails.
The rectangular shape of the lenticular chambers is advantageous in keeping elastomeric O-rings located at the longitudinal ends of the chamber farther away from the center of the chamber where the wafer is located. These O-rings have a tendency to heat up, and therefore, if located too close to the susceptor/wafer combination at the center of the chamber, they will become difficult to cool and may burn more easily due to exposure to high temperatures. Moreover, a rectangular shape evenly distributes gas flow through the chamber. By providing a longer longitudinal distance for gas to flow over the wafer to be processed, the gas can spread out in the chamber before reaching the wafer, thereby allowing a more uniform deposition.
While these lenticular chambers present a good design for low pressure applications, scaling this design to larger sizes presents difficulties. A lenticular chamber designed to accommodate a 200 mm wafer has a length of about 600 mm, a width of about 325 mm, and a chamber height of about 115 mm. To increase the chamber size for a 300 mm wafer, while maintaining relatively the same rectangular proportions, the chamber would have to have a length of about 900 mm and a width of about 488 mm. Such a chamber is big and heavy, and more difficult to fabricate, requiring special cranes and lifting devices. The increased footprint also decreases the amount of clean room space available. Furthermore, the larger size makes the chamber more difficult to clean.
Lenticularly-shaped chambers could also be improved to favor a more uniform deposition of material. In such chambers, the quartz wall disposed over the wafer to be processed is curved, creating a greater chamber volume above the center of the wafer than over the lateral edges, such that uniform deposition is difficult to achieve.
Deposition uniformity is affected by the gas flow profile produced over the wafer, both in lenticular and other types of chambers. There have been attempts to control the gas flow profile in parallel across the wafer to be processed, to create a more uniform deposition. For example U.S. Pat. No. 5,221,556 discloses a system in which the apertures of the gas inlet manifold are varied in size to allow relatively more gas through a particular section, typically the center section. U.S. Pat. No. 5,269,847 includes valves for adjustment of pairs of gas flows merging into a number of independent streams distributed laterally upstream of the wafer to be processed. This system emphasizes the importance of channeling the various gas flows separately until just before the wafer leading edge in order to prevent premature mixing and enable greater control over the flow and concentration profiles of reactant and carrier gases across the wafer.
Despite recent advancements, a need still exists for a processing chamber with an improved design. Preferably, such a chamber should exhibit uniform deposition. At the same time, the chamber should be lightweight and compact, but still able to withstand pressure differentials and high temperatures, particularly for wafers 300 mm and larger. Furthermore, this chamber should be made lightweight and strong without subjecting the chamber to depositions or cracking due to thermal effects.